A lot will depend on if you need a payment break plus how long they are giving you to repay it.
IVA started March 2011, Completed March 2016 and certificate issued 11 days after final payment. It was not always easy but then some of the best decisions aren't.
Your IP should assist in allowing the repayment amount, if paying over time, as part of your expenses, or, as lifenoteasy suggests, a payment break to pay this off. HMRC might allow payment by reduction of ongoing allowances.
Either way your income will be reduced, so a fresh I&E might be worthwhile.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014
If you have been receiving too much in tax credits then your IVA payments have been too high. Mkae sure this is reflected in any payment reduction or break as an extension would be unfair.